The present invention relates to an intermediate transfer unit used in an image formation apparatus using an electrophotographic method, such as a copying machine, a printer and a facsimile. The present invention also relates to a recording medium carrier system applied to the image formation apparatus.
As for a copying machine, a printer, a facsimile and other image formation apparatuses respectively using electrophotography, above all, an image formation apparatus using a laser beam writing device, a function for transferring and fixing a toner image while carrying a recording medium at high speed is required to make good use of the function of the writing device, and operability for allowing a simple measure for paper jam and others caused by the provision of such a function is also required.
Generally, an image formation apparatus using electrophotographic technology is provided with a photoconductive drum provided with a photosensitive layer as the peripheral face, charge means for evenly charging the peripheral surface of the photoconductive drum, exposure means for selectively exposing the peripheral surface evenly charged by the charge means to form an electrostatic latent image, developing means for applying toner as a developer to the electrostatic latent image formed by the exposure means to form a visible image (a toner image), and transfer means for transferring the toner image developed by the developing means on a transfer medium such as paper.
For transfer means for transferring a toner image developed on a photoconductive drum on a transfer medium such as paper, heretofore, there is known transfer means provided with an intermediate transfer belt to which a toner image formed on a photoconductive drum is transfered (primary transfer) and which further transfers (secondary transfer) the toner image on a recording medium, and with a driving roller for circulating the intermediate transfer belt.
As for the above prior transfer means, there is a problem that since distance between a primary transfer position and the driving roller is large, the amount of shrinkage of the intermediate transfer belt between them is increased, the travel speed of the intermediate transfer belt in the primary transfer position is unstable, and as a result, it is difficult to acquire satisfactory primary transfer.
Further, according to the above prior transfer means, there is a problem that a transfer roller is directly touched to the joint of the intermediate transfer belt, a secondary transfer roller is stained by toner accumulated in a step of the joint of the intermediate transfer belt, and toner adheres to the rear of a recording medium in the next secondary transfer.
Further, according to the above prior transfer means, there is a problem that when a thin line image is transferred on a recording medium the surface of which is smooth, the failure of the transfer of toner (a void) occurs.
Further, according to the above prior transfer means, there is a problem that even if transfer on a recording medium the surface of which is smooth is satisfactory, transfer on a recording medium the surface of which is rough is insufficient and particularly, when multiple layers of toner is transferred as a multiple color image, the failure of transfer of toner of a layer far from the surface of a recording medium occurs.
Further, according to the above prior transfer means, there is a problem that in primary or secondary transfer, the deterioration of transfer efficiency and the omission (void) of a part of a toner image in transfer occurs. Also, in secondary transfer, there is a problem that it is difficult to transfer on a recording medium the surface of which is extremely irregular such as recycled paper and bond paper without lacking a part of an image. There is a problem that particularly, if toner the fluidity of which is high is used, toner is readily scattered in transfer, particularly, if primary or secondary transfer means which functions as a transfer electrode for applying transfer voltage to a transfer position is located in a position distant from its transfer position, a transfer electric field in the transfer position cannot be concentrated upon the transfer position, a toner image is scattered due to electrostatic force and if for example, the intermediate transfer belt is wound on the photoconductive drum without means for substantially pressing the intermediate transfer belt on the photoconductive drum or a recording medium in a transfer position, area in which the photoconductive drum and the intermediate transfer belt are in contact in a transfer position is large and the turbulence of a toner image due to mechanical force caused by slight difference in speed between both and others readily occurs.
Further, according to the above prior transfer means, a monolayer or multilayer belt in which a conductive, a semiconductive or an insulating resin layer is generally formed at least as the surface layer, is used for the intermediate transfer belt. Thus, there is a problem that since the surface is made of resin as described above, friction and a scratch are readily generated. Particularly, a large quantity of particulates of metallic oxide generally adhere to the surface of a toner particle as an additive, and there is a problem that since the above additive is extremely harder than resin constituting the surface of the intermediate transfer belt, it is readily embedded in the intermediate transfer belt, further a phenomenon (so-called filming) in which toner adheres to the intermediate transfer belt in the above embedded point occurs and the deterioration of an image, for example the deterioration of transfer efficiency in primary or secondary transfer and the lack of a part of a toner image in transfer (void) occurs. Also, in secondary transfer, there is a problem that it is difficult to transfer on a recording medium the surface of which is extremely irregular, such as recycled paper and bond paper, without causing the imperfection of an image.
Further, according to the above prior transfer means, there is a problem that a phenomenon that a part of a toner image transferred on the intermediate transfer belt in primary transfer, particularly the center lacks, a so-called void occurs. Also, in secondary transfer, there is a problem that it is difficult to also transfer on a recording medium the surface of which is extremely irregular, such as recycled paper and bond paper, without causing an imperfect image in addition to the above problem of a void. Further, in an image formation apparatus for forming a full color image by overlapping plural colors for example, secondary transfer means is prevented from being stained by controlling the driving of the secondary transfer means for executing secondary transfer so that the means is not in contact with the intermediate transfer belt while images of each color are formed and is touched to the intermediate transfer belt after the final image is formed, and when secondary transfer is started before primary transfer is finished, an image on the intermediate transfer belt is prevented from being disturbed. However, there is a problem that the intermediate transfer belt is vibrated, the speed is varied, and the turbulence of an image occurs when the state of the secondary transfer means is switched to a state in contact or not in contact with the intermediate transfer belt.
Further, according to the above prior transfer means, transferability in a primary transfer part is insufficient. Concretely, there are problems in the quantity of toner (the thickness of the layer), dispersion in resistance among each member, the variation of transfer efficiency due to the variation of resistance, a phenomenon of a void, and the stability of the density due to aging.
Further, according to the above prior transfer means, transferability in a secondary transfer part is insufficient. Concretely, there are problems in the quantity of toner (the thickness of the layer), the type of a recording medium such as plain paper, a postal card, and OHP sheet, dispersion in resistance and the variation of resistance among each member, the variation of transfer efficiency due to the variation of resistance by environment, a phenomenon of a void, and the stability of the density due to aging.
Further, in the above prior transfer means, with respect to resistance which is the important characteristic of a primary transfer member and a secondary transfer member, members having approximately the same variation of resistance due to environment are used for both the primary and secondary transfer members.
Therefore, if a member having small variation of resistance due to environment is used for both, current may leak in a part not related to transfer and the failure of transfer may occur in case a recording medium such as a postal card and an envelope smaller in size than the width of the secondary transfer member is printed in the environment of low temperature and low humidity in which the resistance of the recording medium is higher than that of the secondary transfer member in a secondary transfer part. To avoid the above situation, it is conceivable to increase the resistance of the secondary transfer member and reduce leakage current. However, since a member having small variation of resistance due to environment generally has the large dispersion of the resistance, there is a problem that the nonuniformity of transfer partly occurs.
In the meantime, if a member having large variation of resistance due to environment is used for both, no failure due to a leak of secondary transfer occurs because the resistance of the secondary transfer member changes approximately as the change of the resistance of a recording medium due to environment. However, voltage required in a primary transfer part in the environment of low temperature and low humidity causes the increase of the cost.
Further, in a prior transfer means as disclosed in Japanese Patent Application No. Hei. 7-322667, an imperfect image is prevented from occurring at the simultaneous timing of primary transfer and secondary transfer by providing a conductive layer to the intermediate transfer belt and setting relationship between resistance RT of a part from a primary transfer bias applying power source to the conductive layer and apparent resistance R1 in a primary transfer part so that RT<R1.
According to above prior transfer means, there is a case that it is insufficient, depending upon environment and the type of paper, to prevent an imperfect image from occurring at the simultaneous timing of primary transfer and secondary transfer. Concretely, if current which flows in secondary transfer is larger than current which flows in primary transfer, the phenomenon is remarkable.